Internet consists mostly of wired data communication equipment To - - PowerPoint PPT Presentation

The robust hardware

• f Internet

To build a stable global wide area networks is a major electrical challenge!

Internet consists mostly of wired data communication equipment

William Sandqvist william@kth.se

How do you fight the electrical interference?

Ethernet contact

William Sandqvist william@kth.se

Ethernet contact contains:

• Pulse transformer
• Common Mode Choke

Isolatin with transformer

William Sandqvist william@kth.se

Ground another Ground

The transformers isolate different devices from each other even if the ground potentials should happen to differ by up to 1000 V! upp till 1000 V!

Signals and Interference

William Sandqvist william@kth.se

• Signals are differential mode, they hawe different current direction in

the they have different current direction in wire and return wire.

• Interference are common mode, they have been generated from
• utside and has the same current direction in both wires.

All wires has capacitance against the earth (ground). Signals can ”leak” from the wires to ground.

Signals are differential mode DM Interference are common mode CM

Balanced equipment

William Sandqvist william@kth.se

Commonmode-interference has no impact on signal if it we have a balanced equipment.

Balanced = same leakage to ground.

Signals are differential mode DM Interference are common mode CM

Unbalanced equipment

William Sandqvist william@kth.se

An unbalanced equipment has "closer" to the ground for one conductor, a part of the interference then takes that way – the remaining rest of the interference is perceived as if it were the signal. Balanced equipment is, therefore preferable.

Unbalanced = different leakage to ground.

Some part of the interference remains and will be interpreted as a signal!

Signals are differential mode DM Interference are common mode CM

Signal+Interference!

Which instrument suppress common mode interference?

William Sandqvist william@kth.se

10000 kr 100 kr

Which instrument suppress common mode interference?

William Sandqvist william@kth.se

• The expensive oscilloscope has grounded inputs, thus becoming sensitive

to common mode interference!

• The cheap handheld instrument is

isolated from ground and is completely insensitive to common mode voltages!

Balanced pulse transformer

William Sandqvist william@kth.se

The transmitted data signal consists of two "opposing" pulses which together run a current through both winding halves. Local interference occurs common mode and will instead be led to ground, and never reach the transformer's secondary side. A balanced DM-signal ( with respect to ground ) The result is an isolated and balanced DM signal relieved of "local" interferece.

0-1-0 1-0-1

Common mode choke

CM-Choke

• Interference, common-mode current ( = parallel connected coils ),

meets inductance and is retarded

• Signal, differential-mode current ( = series connected coils ),

meets hardly any inductance at all!

0,9 0,9 2 0,9 0,2

TOT

k M L L L L L L =

• =

= + − ⋅ = ≈

2

(0,9 ) 0,19 0,9 0,9 2 0,9 0,2

TOT

L L L k M L L L L L L L ⋅ − =

• =

= = ≈ + − ⋅

William Sandqvist william@kth.se

M L L M L L LPAR 2

2 1 2 2 1

− + − ⋅ = M L L LSER 2

2 1

− + =

Twisted pair cable

William Sandqvist william@kth.se

+

• +
• = 0

A twisted pair cable becomes insensitive to magnetic interference from outside, resulting induced emf becomes≈0!

• Between equipment twisted pair cable is used.

The wires are tightly connected to each other k ≈1.

• The wire inductance get ≈0

for signal, but ≠ 0 for the interference!

2 ≈ − + = L L L LTOT

Magnetic interference!

shielded cable?

William Sandqvist william@kth.se

A shielded cable is immune to electrical interference/field – as a Faraday cage.

• Against radio frequent interference can a shielded Ethernet

cable be used.

Ltake it easy, he will survive … Foil shield

The entire data transfer

William Sandqvist william@kth.se

TX RX Up to 1000 V!

Pulse-transformer CM-choke Twisted pair CM-choke Pulse-transformer

Ethernetfilter as a component

William Sandqvist william@kth.se

1000BASE-CX Gigabit Ethernet for twisted pair cable. Contains both Pulse transformer and CM-Choke Ethernet filter can be purchased as a finished component, mounted in the contact.

Data through a transformer?

William Sandqvist william@kth.se

e + e t

Data may not be constant for any length

• f time because ”direct voltages" can not

pass through transformers! 1 1 1 1 1 1 1

• Therefore, data is "Manchester Coded".

Manchester Code

William Sandqvist william@kth.se

( Do you remember the gates from Digital Design? What do you think is inside the Manchester coder? ) Manchester encoding implies that 1 and 0 are represented by positive and negative edges. The signal change all the time even when the data is constant and can therefore pass through transformers!

?

Manchester coder

( Manchester coder )

William Sandqvist william@kth.se

It is sufficient with a single exclusive OR gate for producing a Manchester signal from the data and clock pulses!

William Sandqvist william@kth.se

( Manchester decoder )

• With a sequential circuit on the receiving end one can then extract both the

clock signal Ctx and the signal Data from the manchestersignal Man.

• Both sides have their own local clock pulses need not be exactly equal.

Sequence circuit for the manchester decoder is a bit too complicated to be part of a digital design written exam!

Ctx2 Ctx1

Ethernet contact

William Sandqvist william@kth.se

Spare wires?

• two devices can share a single cable (100Mb/s),
• r
• higher transmission speed (1000Mb/s) !

4 signal pairs, two spare wires. What can you use the spare wires to?

Power over ethernet

William Sandqvist william@kth.se

Silly to use "spare wires" (Spare pair) to power - it is better to use them for data transfer (1000Mb/s).

Spare wires? • power over ethernet

Power over ethernet

William Sandqvist william@kth.se

• The ”smart” way that does’nt use the ”spare pairs” …

William Sandqvist william@kth.se